The objective of this proposal is to identify factors that regulate estradiol synthesis in developing ovarian follicles. Estradiol is the dominant steroid product of developing preovulatory follicles and continued estradiol secretion is a hallmark of ovulatory, but not atretic, follicles. Recent reports by a number of investigators have provided support for a two cell, two gonadotropin model for the regulation of estrogen biosynthesis in preovulatory follicles. There is evidence that LH stimulates theca cells to secrete androgen and that FSH stimulates granulosa cells to aromatize androgen to estradiol. This proposal is designed to investigate factors that may interfere with or modulate follicular estradiol production - i.e. factors that may cause follicles to become atretic rather than ovulatory. Ovarian progesterone concentrations are high during several reproductive states in which ovulation does not occur (i.e. the luteal phase, pregnancy, pseudopregnancy). We propose to determine the effects of various concentrations of progesterone on the in vitro production of estradiol and androgen by granulosa and theca cells, respectively, isolated from rat follicles. If progesterone has significant effects on follicular estrogen and/or androgen production, additional experiments are planned which include preliminary experiments to determine the mechanism of action of progesterone and experiments aimed at establishing the physiological significance of the results. In a similar manner, prolactin, which is present in plasma in high concentrations during the partially anovulatory states of lactation and hyperprolactinemia, will be tested for effects on theca cell androgen production and granulosa cell secretion of estradiol in vitro. A third series of experiments will explore the steroidogenic capabilities of theca and granulosa cells from cystic follicles of cows. Two types of cysts will be induced experimentally in heifers. Examination of factors and conditions which may depress follicular estradiol secretion will lead to a better understanding of the conditions necessary for the growth and development of ovulatory follicles.